Active implantable medical devices for vagal stimulation generally include a generator implanted in a patient and a lead that is electrically connected to the implanted generator. The lead carries at its distal end one or more electrodes that are placed in contact with the vagus nerve of the patient at the carotid artery to deliver electrical stimulation pulses with a controlled and adjustable energy level. Vagus nerve stimulation by electrical pulses provides control of the blood pressure of the patient, thus it may be used as a treatment choice for hypertension when the treatment of hypertension by medication is deemed ineffective or not suitable.
One type of known vagal stimulation device is described in U.S. Pat. No. 7,123,961 B1. The generator is equipped with an energy management system for stimulating the vagus nerve of a patient and managing and controlling various parameters such as the patient's myocardial contractility (i.e., inotropy), heart rate, and/or atrioventricular conduction (i.e., dromotropy). These parameters are controlled according to the results of an analysis on the components of the patient's cardiac rhythm, including the heart rate, the PR interval, and the QRS complex duration.
U.S. Patent Publication Nos. 2008/0125827 A1, 2008/0119898 A1 and 2006/0111668 A1 describe similar vagal stimulation devices.
The starting point for the present invention stems from clinical observations that hypertension that cannot be adequately treated by medication is prevalent among patients with heart failure, especially those suffering from “diastolic heart failure”. In these patients, the function of the left heart is preserved, meaning that the contractility of the left ventricle is not affected, and the heart failure in such patients is linked to a poor filling capacity. For this reason, diastolic heart failure is also referred to as heart failure “with preserved systolic function”. The pathology of diastolic heart failure is characterized by an abnormally reduced filling phase during the diastole because these patients have left ventricular hypertrophy that provides inadequate diastolic filling.
The presence of hypertension is an additional barrier to optimal diastolic filling. However, if hypertension is treated by vagal stimulation, its effectiveness is constrained to be dependent on the stimulation energy level; the higher the voltage (energy level) delivered by the device, the larger the reduction in the blood pressure. However, strong energy stimulation pulses significantly limit the lifetime of the implantable medical device due to excessive power consumption.